The present invention relates to a continuous method for the preparation of heat-vulcanizing silicone rubber compounds.
Heat-vulcanizing silicone rubber compounds comprised of polydiorganosiloxane gum, inorganic filler, and an optional processing aid are known as base compounds for the fabrication of silicone rubber moldings. Such heat-vulcanizing silicone rubber compounds are generally supplied by silicone producers to silicone rubber molding operations, where they are first blended with curing agent, typically an organoperoxide, and then molded or formed into silicone rubber moldings.
These heat-vulcanizing silicone rubber compounds have been prepared by the introduction of their constituent components into a large mixer equipped with two mixing blades (kneader mixer), followed by mixing while heating.
However, the high-productivity manufacture of a homogeneous, time-stable heat-vulcanizing silicone rubber compound by this prior preparative method is quite problematic. Specifically, the aforesaid prior preparative method frequently yields a silicone rubber compound that is very prone to plasticity retrogression (crepe hardening with elapsed time). Mixing for long periods of time while heating at high temperatures has been necessary in order to avoid this. For example, heating and mixing for 6 to 48 hours is necessary in order to prepare a homogeneous heat-vulcanizing silicone rubber compound that is free of time-dependent changes. In addition, there are limits on the ability to scale-up the kneader mixer itself.
Japanese Patent Application Laid Open [Kokai or Unexamined] Number Hei 2-102007 [102,007/1990] proposes a method for the continuous preparation of silicone rubber compounds in which the constituent components of a heat-vulcanizing silicone rubber compound are converted into a particulate mixture and this particulate mixture is then fed to a twin-screw continuous compounding extruder. This method, however, requires the execution of a complicated step in which the polydiorganosiloxane gum, inorganic filler, and processing aid are temporarily converted into said particulate mixture through the use of a means such as a high-speed mixer, etc. Thus, this method cannot be viewed as having an intrinsically excellent productivity.
The inventors achieved the present invention as a result of extensive research directed at solving the problems described above. The present invention takes as its object the introduction of a method for the continuous production of a time-stable heat-vulcanizing silicone rubber compound with excellent roll workability.